1. Field of the Invention The present invention relates generally to a planar array type microwave antenna for use in receiving, for example, a satellite broadcast and more particularly to a microwave antenna structure.
2. Description of the Prior Art
In the art, a suspended line feed type planar array antenna has been proposed in which a substrate is sandwiched between metal or metallized plastic plates having a number of spaced openings forming a part of radiation elements, operating as a circular polarized wave planar array antenna, in which a pair of excitation probes which are perpendicular to each other, the number of which corresponds to the number of spaced openings, are formed on a common plane and the signals fed to the pair of excitation probes are mixed in phase within the suspended line (in our co-pending U.S. patent applications Ser. No. 888,117 filed on July 22, 1986 and Ser. No. 058,286 filed on June 4, 1987.
Thus, the above-mentioned planar antenna can be reduced in thickness and its mechanical configuration can be simplified. Further, though an inexpensive substrate available on the market is employed as a high frequency use, an antenna gain equal to or larger than that of the planar antenna using an expensive microstrip line can be achieved.
The suspended line can achieve its advantage that it forms a low loss line as a circuit for feeding the planar antenna and also it can be formed on an inexpensive film-shaped substrate, and so on. Further, since this conventional planar antenna utilizes a circular or rectangular wave-guide opening element as a radiation element, it is possible to construct an array antenna which has a small gain deviation over a relatively wide frequency range.
Meanwhile, a so-called patch type microstrip line antenna has been proposed, in order to reduce the thickness of the planar array antenna. Also, this patch type microstrip line antenna can be made high in efficiency and wide in bandwidth by effective use of the suspended line and the thin radiation element, and it can be reduced in thickness and in weight at the same time, as is disclosed in our co-pending U.S. patent application Ser. No. 223,781 filed on July 25, 1988 and Ser. No. 258,728 filed Oct. 7, 1988.
In a suspended line feed type planar array antenna in which a substrate is sandwiched between a pair of metal or metallized plastic plates, the resonance type printed patch radiators are formed on the substrate at positions corresponding to slots formed through one of the metal or metallized plastic plates.
In the planar antenna in which the substrate is sandwiched between the pair of plates, a protective cover, sometimes called a radome, is generally used to protect an antenna body formed of a pair of plates and a substrate from wind, snow or rain. FIG. 1A illustrates such an antenna. In the antenna shown in FIG. 1A, there are shown a radome 1 and a rear cover 2 between which an antenna body 3 is provided as shown in FIG 1. A packing 4 is inserted between the radome 1 and the rear cover 2 to provide a waterproof property, as shown in FIGS. 1A and 1B. In order to fix the radome 1, the rear cover 2 and so on, two protective trims 5 of C-shaped cross section are engaged with the radome 1, the rear cover 2 and so on from the lateral direction, as shown in FIG. 1C. Finally, these protective trims 5 are fastened to the rear cover 2 by screws 6.
In the conventional planar antenna shown in FIGS. 1A to 1C, a through-hole 7 (FIG. 1B) must be formed from the side surface of the rear cover 2 so that the mold structure becomes complicated, the productivity thereof is poor and the cost thereof is increased.
Further, since the protective trims 5 are fastened to the rear cover 2 by the screws in the lateral direction, substantially no force is applied to the end portions of the radome 1 and the rear cover 2 in the up and down direction. Thus, the packing 4 cannot be expected to have sufficient waterproof property.
The two protective trims 5 grip the radome 1 and the rear cover 2. In this case, as shown in FIG. 1C, the length of the protective trim 5 is increased so that a large draft is needed to mold the protective trim 5. Thus, the protective trims 5 cannot be formed to have a uniform C-shaped cross-section so that the protective trim 5 cannot be fastened to the rear cover 2 without play. Also, the through-hole 7 for the screw 6 must be formed through the side portion of the rear cover 2 so that the mold for molding the protective trim 5 becomes complicated in structure.
Further, since the radome 1 and the rear cover 2 are secured by the protective trim 5 by means of screws 6, tools are needed in the assembly and the efficiency in the assembly process is sluggish.